System for Switching from an Established Media Path to a New Media Path in a Packet Based Network

ABSTRACT

There is described a system for switching from an established media path to a new media path for packets transmitted between first and second end points in a packet based network. An instruction is transmitted to the first end point instructing it to set up a new receiving channel for receiving packets transmitted from the second end point over the new media path. An instruction is transmitted to the second end point instructing it to configure a sending channel to send packets over the new media path for reception on the new receiving channel of the first end point. For a time period during the switching, the first end point monitors for packets simultaneously on both its new receiving channel and an established receiving channel configured to receive packets transmitted from the second end point over the established media path. This systems helps ensure a seamless switchover from the established to the new media paths.

This invention relates to seamless media flow switching in a packetswitched network.

Telecommunications has evolved towards packet based communications. Apacket switched network is a network in which units of data known aspackets are routed through a network based on the destination addresscontained in each packet. Packet switched networks are connectionlessnetworks in which each packet in a data flow is routed independentlyfrom the other packets in that data flow to the destination. It is quitepossible for different packets in the same data flow to be routed bydifferent routes to the destination. In contrast, a circuit switchednetwork, for example a traditional telephone system, is a connectionorientated network, in which a physical path is obtained for anddedicated to a single connection between two end-points in the networkfor the duration of the connection.

The term Voice over IP (VoIP) denotes the provision of voice/audioservices over packet switched networks using the Internet Protocol (IP).A number of international data network standards support VoIP includingITU-T H.323 and the Session Initiation Protocol (SIP).

H.323 is a protocol stack that supports VoIP by referencing specificprotocols for speech coding, call setup, signaling, data transport andother areas rather than specifying these things itself. For example, theReal Time Protocol (RTP) managed by the Real Time Control Protocol(RTCP) is used for actual data transmission.

Whereas H.323 is a complete protocol suite, SIP is a single module anddescribes how to set up VoIP sessions. SIP just handles setup,management and termination of sessions. Other protocols such as RTP/RTCPare used for data transport. SIP is an application layer protocol andcan run over User Datagram (UDP) protocol or Transmission ControlProtocol (TCP).

In some circumstances it is necessary or desirable to re-route a mediastream between two end points in a packet switched network. For example,in prior art systems re-routing a media stream is achieved using eitherthe H.323 pause and re-route feature, the H.323 extended fast connectfeature or the SIP offer/answer feature. None of these features providea seamless method of payload switching. Use of all these features tore-route a payload stream between two end points results in the payloadstream being interrupted for a short time before being re-connected.This may cause packets to be lost.

Embodiments of the present invention aim to alleviate this problem.

According to the present invention there is provided a system forswitching from an established media path to a new media path for packetstransmitted between first and second end points in a packet basednetwork, the method comprising: transmitting an instruction to the firstend point instructing it to set up a new receiving channel for receivingpackets transmitted from the second end point over the new media path;transmitting an instruction to the second end point instructing it toconfigure a sending channel to send packets over the new media path forreception on the new receiving channel of the first end point; andwherein for a time period, the first end point monitors for packetssimultaneously on both its new receiving channel and an establishedreceiving channel configured to receive packets transmitted from thesecond end point over the established media path.

According to the invention there is also provided a method of operatinga first end point when switching from an established media path to a newmedia path for packets transmitted from a second end point to the firstand in a packet based network, the method comprising: setting up a newreceiving channel for receiving packets transmitted from the second endpoint over the new media path and for a time period, monitoring forpackets simultaneously on both the new receiving channel and anestablished receiving channel configured to receive packets transmittedfrom the second end point over the established media path.

The above and further features of the invention are set forth withparticularity in the appended claims and together with advantagesthereof will become clearer from consideration of the following detaileddescription of exemplary embodiments of the invention given withreference to the accompanying drawings, in which:

FIGS. 1 to 9 illustrate a first packet switched network embodying theinvention;

FIG. 10 illustrates a message flow diagram;

FIG. 11 illustrates another message flow diagram;

FIGS. 12 to 20 illustrate a second packet switched network;

FIG. 21 illustrates another message flow diagram.

Turning now to FIG. 1 of the accompanying drawings, a communicationsystem 1 comprises a first VoIP end point 2, a second VoIP end point 3and a Private Branch Exchange (PBX) 4. In this embodiment, the firstVoIP end point 2 and the second VoIP end point 3 are both IP telephones,for example Siemens optipoint™ 400 IP telephones. The switching matrix 5of the PBX 4 is a Time Division Multiplexed (TDM) circuit switchedmatrix and the PBX 4 may for example be a Siemens HiPath™ 4000 system.The first VoIP end point 2 and the second VoIP end point 3 may each beconnected to the PBX 4 via an IP-based Local Area Network (LAN) (notshown).

In this example, the first 2 and second 3 VoIP end points have anestablished two-way voice connection between themselves. Both thesignalling path 6 and the payload path 7 of this voice connection runthrough the PBX 4. As mentioned above, the PBX 4 has a TDM switchingmatrix 5 and so the PBX 4 is provided with Linecard Gateways (not shown)for converting the IP signalling and IP payload flows received at thePBX 4 into a TDM format for transmission across the PBX 4 andsubsequently back into an IP format for transmission away from the PBX4.

The conversion by the PBX 4 of the payload path 7 from an IP flow into aTDM flow and then back again into an IP flow increases both the latencyand the jitter of the voice connection. This is undesirable. In thisembodiment of the invention, after the voice connection between thefirst 2 and second 3 VoIP endpoints becomes stable, a new payload path 8avoiding the PBX 4 is established and the initial payload path 7 isbroken. This illustrated in FIG. 2.

Referring now to FIGS. 3 to 10, the process by which the new payloadpath 8 is established will be described. For simplicity, the signallingpath 6 which throughout this process continues to pass through the PBX4, is not shown in FIGS. 3 to 9.

The initial payload path 7 comprises a forward payload path 7 a and areverse payload path 7 b. The first VoIP endpoint 2 transmits itspayload using send channel S1-A over the forward payload path 7 a toreceive channel R1-B of the second VoIP endpoint 3. In this context achannel may be thought of as being defined by the IP address of therelevant end point and the port number used to transmit or receive dataon. The second VoIP endpoint 3 transmits its payload using send channelS1-B over the reverse payload path 7 b to receive channel R1-A of thefirst VoIP endpoint 2. The payload transmitted from the first VoIP endpoint 2 is received at the PBX 4 on receive channel R1-PBX and istransmitted from the PBX 4 on transmit channel S1-PBX. The payloadtransmitted from the second VoIP end point 3 is received at the PBX 4 onreceive channel R2-PBX and is transmitted from the PBX 4 on transmitchannel S2-PBX.

To initiate establishment of the new payload path 8, the PBX 4 sends arequest to the second VoIP end point 3, step 100, to instruct it toprovide an additional receive channel R2-B. The second VoIP end point 3sets up the additional receive channel R2-B and sends a message back tothe PBX 4, step 101, to inform it that the receive channel R2-B is setup. At this point, the second VoIP end point 3 listens on both receivechannels R1-B and R2-B simultaneously. Next, the PBX 4 sends a messageto the first VoIP end point 2, step 102, to inform it that the receivechannel R2-B of the second VoIP end point 3 is set up. In response, thefirst VoIP end point 2 reconfigures its sending channel S1-A to begintransmitting its payload to the new receive channel R2-B of the secondVoIP end point 3 on new forward payload path 8 a which avoids the PBX 4.Next, the first VoIP end point 2 sends a message back to the PBX 4, step103, to inform it that the sending channel S1-A has been reconfiguredand is sending payload to the new receive channel R2-B. At step 104, thePBX 4 sends a message to the second VoIP end point 3 instructing it toclose the initial receive channel R1-B, which it then does.

The PBX 4 then sends a request to the first VoIP end point 2, step 105,to instruct it to provide an additional receive channel R2-A. The firstVoIP end point 2 sets up the additional receive channel R2-A and sends amessage back to the PBX 4, step 106, to inform it that the receivechannel R2-A is set up. At this point, the first VoIP end point 2listens on both receive channels R1-A and R2-A simultaneously. Next, thePBX 4 sends a message to the second VoIP end point 3, step 107, toinform it that the receive channel R2-A of the first VoIP end point 2 isset up. In response, the second VoIP end point 3 reconfigures itssending channel S1-B to begin transmitting its payload to the newreceive channel R2-A of the second VoIP end point 2 on new reversepayload path 8 b which avoids the PBX 4. Next, the second VoIP end point3 sends a message back to the PBX 4, step 108, to inform it that thesending channel S1-B has been reconfigured and is sending payload to thenew receive channel R2-A. At step 109, the PBX 4 sends a message to thefirst VoIP end point 2 instructing it to close the initial receivechannel R1-A, which it then does.

FIG. 11 illustrates an alternative flow of messages that may be sentfrom the PBX 4 to the first 2 and second 3 VoIP end points using H.460.6to implement an embodiment of the invention.

In order to be able to synchronise the payload packets on the old andnew payload path, e.g. if there are different message delivery durationsfor the voice packets travelling through the network, the receiving endpoint has to be able to re-order the received packets. To allow theendpoint to re-order packets a packet counter can be realized, e.g. EP-Asends packet number seventeen towards R1-B and then switches sendingpacket number eighteen towards R2-B. EP-B receives packet 18 on R2-Bbefore packet 17 on R1-B. Because EP-B knows that the last packetreceived was packet number sixteen, EP-B is able to re-order the packetsin the correct sequence.

Realization of such a synchronization method can be done followingseveral approaches, for example, using Silence Insertion Descriptor(SID) (if SID is supported) together with a flag bit in the RTP header.As is well known, SID frames are inserted into audio streams to include“comfort noise” during periods of silence. In the above describedexample, the next occurrence of a SID frame following the set up of thenew media stream could be used as the synchronisation point forswitchover. A SID frame would be inserted on the old payload path beforeswitching over to the new payload path. On the new payload path also aSID frame would be sent, before transmission of the payload packetsstarts. This would allow the receiving end point to synchronise the twopayload steams.

Alternatively, Real Time Protocol Control Protocol (RTCP) sender reports(SR), which contain statistics such as timestamps and packet sequencenumbers and source description (SDES) items that explicitly identify thesource of audio packets may be used to compute the synchronisation pointfor switching to the new media path.

Other such proprietary protocol extension may also be used for thispurpose.

A further embodiment of the invention will now be described with respectto FIGS. 12 to 22.

Turning first to FIG. 12, a communication system 20 comprises a VoIP endpoint 21, a Wireless Local Area Network (WLAN) user terminal 22, a firstWLAN access point 23, a second WLAN access point 24 and a WLANcontroller 25. In this embodiment, the first VoIP end point 21 is againan IP telephone such as the Siemens optipoint™ 400 IP telephone.

In this example, the VoIP end point 21 and the WLAN user terminal 22have an established two-way voice connection between themselves. TheWLAN user terminal 22 is being serviced by the first WLAN access point23. A connection oriented signalling path 26 runs between the VoIP endpoint 21 and the WLAN controller 25, and between the WLAN controller 25and the WLAN user terminal 22 via the first WLAN access point 23. Apayload path 27 runs between the VoIP end point 21 and the WLAN userterminal 22 via the first WLAN access point 23.

In this embodiment of the invention, the WLAN user terminal 22 movesfrom the region served by the first WLAN access point 23 to the regionserved by the second WLAN access point 24. A handover of the WLAN userterminal 22 from the first WLAN access point 23 to the second WLANaccess point 24 occurs. During the handover, a new payload path 28 isestablished between the VoIP end point 21 and the WLAN user terminal 22via the second WLAN access point 24 and a new connection orientedsignalling path 29 is established that runs between the WLAN controller25 and the second WLAN access point 24 and between the second WLANaccess point 24 and the WLAN user terminal 22. The connection orientedleg of the signalling path between VoIP end point 21 and the WLANcontroller 25 may stay unchanged. This is illustrated in FIG. 13. Inaddition to the connection oriented signalling path between the WLANcontroller 25 and the WLAN user terminal 22 (traversing either the WLANaccess point 23 or 24), connection less signalling paths between theWLAN controller 25 and the WLAN access points 23 and 24 exist, allowingWLAN controller and access point communication. This allow the secondWLAN access point 24 to inform the WLAN controller 25, that a WLAN userterminal has arrived in the covered service area. For simplicity, thissignalling path is omitted in the figures.

Referring now to FIGS. 14 to 22, the process by which the new payloadpath 28 is established will be described. For simplicity, no signallingpaths are not shown in FIGS. 14 to 20.

The initial payload path 27 comprises a forward payload path 27 a and areverse payload path 27 b. The VoIP endpoint 21 transmits its payloadusing send channel S1-A over the forward payload path 27 a to receivechannel R1-AP1 of the first WLAN access point 23. The first WLAN accesspoint 23 transmits the received payload using transmit channel S2-AP1over the air using the forward payload path 27 a to receive channel R1-Bof the WLAN user terminal 22. The WLAN user terminal 22 transmits itspayload using send channel S1-B over the air using the reverse payloadpath 27 b to receive channel R2-AP1 of the first WLAN access point 23.The first WLAN access point 23 transmits this received payload usingtransmit channel S1-AP1 over the reverse payload path 27 b to receivechannel R1-A of the VoIP endpoint 21.

At step 200, the second WLAN access point 24 sends a message to the WLANcontroller 25 informing it, that the WLAN user terminal 22 has arrivedin the service area covered by the second WLAN access point 24. The WLANcontroller 25 then sends a Setup message to the second WLAN access point24 requesting the establishment of a communication session between theWLAN controller 25 and the WLAN access point 24 on behalf of WLAN userterminal 22. The WLAN access point 24 responds with a Connect messageconfirming the establishment of the communication session in step 202.

Next, in step 203 the WLAN access point 24 is requested by the WLANController 25 to add a new receive channel R1-AP2 for receiving payloadfrom the transmission channel S1-A of the VoIP end point 21 over the newforward payload path 28A. In step 204 the WLAN access point 24 instructsthe WLAN user terminal 22 to add a new receive channel R2-B and to beprepared to receive media on this channel. The WLAN user terminal 22then responds to the second WLAN access point 24, step 205, confirmingthe set up of the new receive channel. Receiving this confirmation, thesecond WLAN access point 24 establishes a new transmission channelS2-AP2 towards the receive channel R2-B of the WLAN user terminal 22 toset up the new forward payload path 28A.

After setting up the new receive channel R1-AP2 and the new transmissionchannel S2-AP2, the second WLAN access point 24 sends a message to theWLAN controller 25 in step 206, informing it that these channels are setup. At step 207, the WLAN controller sends a message to the VoIP endpoint 21 requesting that it re-configures its transmission channel S1-Ato transmit payload to the receive channel R1-AP2 of the second WLANaccess point 24 for onwards transmission from the transmission channelS2-AP2 of the second WLAN access point 24 to the receive channel R2-B ofthe WLAN user terminal 22. The VoIP end point 21 re-configures itstransmission channel S1-A accordingly, and payload is transmitted on thenew forward payload path 28A.

At step 208, the VoIP end point 21 sends a message to the WLANcontroller 25 informing it that the transmission channel S1-A has beenre-configured, and at step 209, the WLAN controller 25 sends a messageto the first WLAN access point 23 requesting it to close its receivepath consisting of R1-AP1 and S2-AP1, which it does. In addition theWLAN controller 25 could inform the WLAN user terminal 22 to close thereceive channel R1-B, which is no longer needed, although such a step isnot illustrated in FIG. 21.

In general closing R1-AP1, S2-AP1 and R1-B could be also delayed untilsuccessful establishment of the complete payload path, to allow the WLANuser terminal 22 to stay in the region served by the first WLAN accesspoint 23, should the user leaves the region served by the second WLANaccess point 24 again.

At step 210, the WLAN controller 25 sends a message to the VoIP endpoint 21 instructing it to set up a new receive channel R2-A. The VoIPend point 21 sets up the new channel and at step 211 sends a message tothe WLAN controller 25 to inform it accordingly. At step 212, the WLANcontroller 25 sends a message to the second WLAN access point 24requesting it to set up a new send channel S1-AP2 for sending payloadtowards the receive channel R2-A of the VoIP end point 21. Next, step213, the WLAN access point 24 creates a receive channel R2-AP2 and sendsa message to the WLAN user terminal 22 requesting it to reconfigure itstransmission channel S1-B to transmit payload to the receive channelR2-AP2 of the second WLAN access point 24 for onwards transmission fromthat access point's transmission channel S1-AP2 to the receive channelR2-A of the VoIP end point 21. At this point in time the new payloadpath 28 is established in forward and reverse direction completely.

After reception of a channel re-configuration result from the WLAN userterminal 22, step 214, the WLAN access point 24 sends a confirmationmessage to the WLAN controller 25, step 215. On receipt of this message,the WLAN controller 25 sends a message to the VoIP end point 21requesting it to close the initial receive channel R1-A, step 216. TheWLAN controller 25 then sends a disconnect message to the first WLANaccess point 23, step 217, which replies with a release message, step218. This closes the signalling connection between the WLAN controller25 and the first WLAN access point 23 on behalf of the WLAN userterminal 22 and the handover is then complete.

Any of the synchronisation methods described with respect to the firstembodiment, may also be used with the second embodiment.

Having thus described the present invention by reference to preferredembodiments it is to be well understood that the embodiments in questionare exemplary only and that modifications and variations such as willoccur to those possessed of appropriate knowledge and skills may be madewithout departure from the scope of the invention as set forth in theappended claims.

1. A method for switching from an established media path to a new mediapath for packets transmitted between first and second end points in apacket based network, the method comprising: transmitting an instructionto the first end point instructing it to set up a new receiving channelfor receiving packets transmitted from the second end point over the newmedia path; transmitting an instruction to the second end pointinstructing it to configure a sending channel to send packets over thenew media path for reception on the new receiving channel of the firstend point; and wherein for a time period, the first end point monitorsfor packets simultaneously on both its new receiving channel and anestablished receiving channel configured to receive packets transmittedfrom the second end point over the established media path.
 2. A systemaccording to claim 1, the system further comprising: transmitting aninstruction to the second end point instructing it to set up a newreceiving channel for receiving packets transmitted from the first endpoint over the new media path; transmitting an instruction to the firstend point instructing it to configure sending channel to send packetsover the new media path for reception on the new receiving channel ofthe second end point; and wherein for a time period during theswitching, the second end point monitors for packets simultaneously onboth its new receiving channel and an established receiving channelconfigured to receive packets transmitted from the first end point overthe established media path.
 3. A method according to claim 1, wherein,at least one of the first and second end points is a VoIP end point. 4.A method according to claim 2 wherein either of the first and second endpoint is able to synchronize media packets received via its establishedreceiving channel and media packets received via its new receivingchannel into one stream.
 5. A method according to claim 4 wherein packetcounters are used to allow end point reordering of media packets.
 6. Amethod according to claim 4 wherein an occurrence of a Silence InsertionDescriptor (SID) frame is used as a synchronisation point by either ofthe first and second end points for switching from the established mediapath to the new media path.
 7. A method according to claim 4 whereinsender reports (SR) of the Real Time Protocol Control Protocol (RTCP)which contain statistics such as timestamps and packet sequence numbersand source description (SDES) are used by either of the first and secondend points for synchronising switching from the established media pathto the new media path.
 8. A method according to claim 4 whereinproprietary enhancements of RTP/RTCP are used by either of the first andsecond end points for synchronising switching from the established mediapath to the new media path.
 9. A method according to claim 1, whereinthe end point supports RTP/RTCP and SIP or H.323.
 10. Apparatus forswitching from an established media path to a new media path for packetstransmitted between first and second end points in a packet basednetwork, the apparatus comprising: means for transmitting an instructionto the first end point instructing it to set up a new receiving channelfor receiving packets transmitted from the second end point over the newmedia path; means for transmitting an instruction to the second endpoint instructing it to configure a sending channel to send packets overthe new media path for reception on the new receiving channel of thefirst end point; and wherein for a time period, the first end pointmonitors for packets simultaneously on both its new receiving channeland an established receiving channel configured to receive packetstransmitted from the second end point over the established media path.11-12. (canceled)